1. Technical Field of the Invention
The present invention relates to an actuator for generating an additional steering angle for road vehicles, having a microprocessor-controlled electric motor functioning as a drive unit, and a gear, which is driven by this electric motor, to apply the additional steering angle to a steered front axle and/or to a rear axle of a vehicle.
2. Prior Art
In conventional steering assemblies for road vehicles like passenger cars, trucks, busses, etc., the steering angle is maintained at the steered axle by means of a mechanically rigid connection from the steering wheel to a steering gearbox and steering tie rods at the wheels. These systems usually employ a steering gearbox that uses a fixed ratio to convert a change in the steering wheel angle into a translatory movement of the steering tie rods. The movement of the steering tie rods, in turn, is converted via steering knuckle arms into the steering angle at the wheels.
In some designs of recent years the steering characteristics of a vehicle are altered by selectively influencing the steering angle. Examples include variable steering reduction ratios, but also control-engineering techniques to improve the dynamics of vehicle movement. In these methods, the characteristics of the dynamics of vehicle movement are measured, or determined by other means, and subsequently compared to corresponding desired values. The difference is then used to determine an appropriate steering angle, as required. The desired values of the dynamics of vehicle movement are generally determined from the steering wheel angle and other parameters, like driving speed, etc., since the steering wheel angle corresponds to the driver""s desires regarding the vehicle""s lateral and yaw dynamics.
With this approach, the obvious solution appears to be a so-called xe2x80x9csteer-by-wirexe2x80x9d solution, whereby a steering angle is calculated in a microprocessor on the basis of all relevant variables, and ultimately transferred by an actuator to the wheels of the steered axle. However, this approach raises considerable safety concerns. Unless appropriate, highly complex safety measures are taken, the vehicle will become unsteerable if a system of the described type malfunctions.
For this reason, the aim in many cases is to retain the mechanical connection between the steering wheel and the wheels of the steered axle and to merely add an additional steering angle mechanically. This additional steering angle results from the difference between a steering angle calculated by a microprocessor, and the steering angle obtained from the position of the steering wheel. A device that is used to perform the mechanical addition of the steering angles will hereinafter be referred to as an auxiliary steering system, for reasons of brevity.
The auxiliary steering system can alternatively be positioned
before the steering gearbox, i.e., in the area of the steering column,
within the steering gearbox, or
following the steering gearbox, i.e., in the area of the steering tie rods or steering knuckle arms.
In case of a failure of the auxiliary steering system, the conventional behavior of the steering assembly is restored simply by blocking the auxiliary steering system, so that the safety of the steering system is not compromised.
The demands placed on auxiliary steering systems vary depending on their intended application. Generally, however, the following characteristics are expected:
a high degree of reliability;
sufficient dynamic characteristics, i.e., a sufficient bandwidth and actuator speed. (The term bandwidth as defined for linear low-pass units describes the frequency at which the logarithmic amplitude response has dropped by xe2x88x923 dB from the horizontal initial amplitude. The bandwidth hence is a measure of up to which frequency an actuator can transfer the required steering angle with sufficient accuracy.)
zero backlash;
little friction;
low construction volume and weight;
low power consumption;
robustness with respect to mechanical loads, like forces, impacts etc.;
depending on the location of its installation: insensitivity to dirt
meet the automobile industry""s demand for low unit cost prices.
For auxiliary steering systems, designs are known, which differ regarding their point of application (see above list), the principle of the mechanical path or angle addition, and the installed actuator type (e.g., hydraulic actuator, electric motor drive).
None of the above designs meets all of the above requirements. If, for example, a hydraulic actuator is used within or following the steering gearbox, a high dynamic bandwidth can be attained only with a very large construction volume and with large line cross sections and the associated high costs. Other versions, in which electric motor driven differential angle gears (planetary gears or differential gears), for example, are installed in the divided steering column, have the disadvantage that this results either in a reversal of the direction of rotation, or in a gear ratio increase that is not equal to one, thus necessitating corresponding compensation measures (re-design of the remaining steering train). These types of gearboxes furthermore bring play and friction into the steering assembly, with the result that the driver""s feeling for the steering control at the steering wheel is rendered less precise.
The present trend in the automobile industry is generally away from the hydraulic systems and toward electrical systems for (power) steering systems, brakes and other auxiliary systems. This is justified by the resulting power savings, since electrical systems require driving power only when an actuation takes place, whereas, in contrast, in hydraulic systems a hydraulic pump runs continually to supply the hydraulic pressure, which needs to be maintained at all times.
Beside the above systems, which are designed to improve the dynamics of vehicle movement by correcting a steering angle that is set by the steering wheel, it has also been suggested to impress an additional steering angle on one of the other vehicle axles, in lieu of correcting the steering angle at the axle that is mechanically steered from the steering wheel. For example, systems are known in otherwise conventionally steered passenger cars, whereby an additional steering angle is applied to the rear axle to reduce the variable angle during cornering.
With these types of systems it is necessary to make the wheels of the corresponding axle steerable. These systems therefore differ from the above systems in that the steering angle set by the steering wheel is not mechanically added to the auxiliary steering angle that is calculated by the microprocessor, but instead only the steering angle calculated by the microprocessor is applied to the respective axle via an appropriate actuator. As far as the use of an actuator is concerned, there is no difference between the systems. The latter design can actually be considered a special case of the former in that merely the steering angle portion that is created mechanically by the steering wheel is eliminated.
In the following discussion, a differentiation is therefore not always made between these two cases. Rather, the terms auxiliary steering angle and/or additional steering angle are used to refer to both, a correcting steering angle that is added to a steering angle applied mechanically by the steering wheel, and a steering angle that is applied to the axle that is not steered mechanically by the steering wheel. Specifically, the same requirements that apply to the first-described auxiliary steering systems also apply to the actuator of the latter design. None of the known versions meets all of these requirements.
The invention is therefore based on the object of creating an actuator for the generation of an additional steering angle for road vehicles, which can be used with different principles of path or angle addition and at different points of application while preventing the shortcomings of the above-described approaches.
In accordance with the invention, this object is accomplished with an actuator for generating an additional steering angle for road vehicles, by using as the gear either a known constant-pitch planetary rolling-contact threaded-spindle gear (constant-pitch PRCTS gear, or a known planetary rolling-contact threaded-spindle (PRCTS) gear, with assigned sensor means with control-engineering type feedback to determine the position of the spindle rod. Advantageous designs, improvements and possible applications of the actuator in accordance with the invention, as well as its use for applying an auxiliary steering angle toan axle that is not steered mechanically by a steering wheel, are also objects of the invention.
With the use according to the invention of the constant-pitch planetary rolling-contact threaded-spindle gear (for the sake of simplicity hereinafter referred to as constant-pitch PRCTS gear) known from DE 195 40 634 (U.S. Ser. No. 08/741,042) or the planetary rolling-contact threaded-spindle gear (for the sake of simplicity hereinafter referred to as PRCTS gear) known from DE 37 39 059 (U.S. Pat. No. 4,926,708), a fast rotary motion of an electric motor serving as the drive unit can be converted at a low torque into an axial movement with very high force, so that the force required to actuate an additional steering angle can optionally be achieved in a single gear step. This use of the PRCTS gear or the constant-pitch PRCTS gear therefore makes it possible to attain very small system increases that cannot be achieved with other spindle types, because of their principle of operation.
The constant-pitch PRCTS gear permits a power transmission at a constant pitch, thus making it possible to draw conclusions regarding the variables of the spindle rod, such as force, position and acceleration, from the known and/or measurable variables of the electric motor and/or motor-driven parts of the spindle gear, such as electric current, angular position, angular velocity, angular acceleration, torque etc.
With the known PRCTS gear, due to its principle of operation, the transmission of the tangential forces from a spindle rod to rollers or rolling elements, and from there to a spindle nut is affected by slip, with the result that the PRCTS gear has a pitch that is not accurately defined and consequently also does not have a constant transmission ratio. For the operation of an auxiliary steering system, however, the additional steering angle and hence the position of the spindle rod of the PRCTS gear must be known. To ensure that the position of the spindle rod can also be set via the electric motor in an accurately defined manner if a PRCTS gear is used, according to the invention the PRCTS gear has an assigned position-sensor means with control-engineering type feedback.
The actuator according to the invention has an installed compact electric motor, which has a low weight and small construction volume and can therefore be installed directly on conventional steering gearboxes or assigned to same. According to a preferred embodiment of the invention, the spindle rod of a PRCTS gear or constant-pitch PRCTS gear concurrently serves as the axle of the electric motor and is installed inside the rotor.
The actuator according to the invention thus has a particularly compact and light-weight design and, specifically, a fixed, constant transfer ratio. Furthermore, the actuator according to the invention consists of only a few components. Lastly, the actuator according to the invention has a high efficiency due to the low occurring friction losses.